This invention relates to a method and an apparatus for operating a steering system for a motor vehicle.
Such a steering system is disclosed by DE-OS 40 31 316 (corresponds to U.S. Pat. No. 5,205,371), and to the extent that it is relevant to understanding this invention, is to be explained by reference to FIGS. 1 and 2. In such a steering system, the motions of the actuator 13 or 23, the motor angle .delta..sub.M, are superimposed in the auxiliary drive 12 or 22 on the steering motions imparted by the driver through the steering wheel 11 or 21, the steering wheel angle .delta..sub.L detected by the sensor 28. The total motion .delta..sub.L ' obtained in this way is transmitted through the steering mechanism 14 and the steering linkage 16 to the steerable wheels 15a and 15b to set the steering angle .delta..sub.V. The actuator 13 or 23 can be an electric motor. The operating principle of such a steering system consists of the motor angle .delta..sub.M being determined to control the dynamic behavior of the vehicle depending on the steering wheel angle .delta..sub.L and depending on signals Sm, with the steering wheel angle .delta..sub.L being determined by the sensor 28 and the signals Sm representing vehicle motions detected by the sensors 26. The total steering angle is found from the equation ##EQU1## in which the transmission ratio can be i.sub.u =1 or i.sub.u .apprxeq.1.
DE A1 36 25 392 shows the emission of a correction signal to a servomotor that controls the front wheel steering angle. The correction signal depends on a desired-actual yaw rate difference.
GB-PS 1,414,206 shows the compensation of crosswind effects by a superimposed steering angle intervention.
DE-OS 40 38 079 (corresponding to U.S. Pat. No. 5,316,379) shows the superimposition of a steering component (compensating steering angle) at the front wheel and/or rear wheel. The compensating steering angle, which depends on the differences in brake pressure, compensates for the yaw motion with so-called .mu.-split brakes, i.e. with brakes in which the road frictions differ significantly on the right and left sides of the roadway. However, there can be the problem in this case that the difference in brake pressures represents only an inaccurate measure of the yaw moment, which can be severely distorted in particular by different left and right tires, fading, nonuniform brake wear, antilock system malfunction, or brake system failure.
In the paper by Ackermann et al.: "Driving Safety From Rugged Steering Regulation", Automatisierungtechnik 44 (1996) 5, pages 219 to 225, it is suggested that the yaw dynamics of a motor vehicle be controlled by steering intervention, with the particular proposal of an integral-control unit.
Although an integrating yaw rate controller with high amplification is particularly suitable for controlling major problems, for example those that occur with .mu.-split antilock brakes or strong crosswind, such a controller on the other hand frequently causes steering interventions that are unnecessary and disturbing in case of small problems, for example such as slightly washboarded roads.
The purpose of this invention consists of improving the yaw behavior of a vehicle by steering interventions, but without bringing about unnecessarily frequent steering interventions.